I, as a user of your software, only care about the feature, and that the feature works. I don't care about testability, whatever that means. And since I case about features, I think this entails that features have to be tested end to end, and not some unrelated classes ("mocks") that are not even used in the final product.

> is easy and makes the resulting code better (eg, easy to replace the notification manager).

It might or it might not. I have seen too much code which was written way too complex which a lot of unnecessary classes and interfaces, just for the sake of "testability". However, some features did not work every once in a while.

If you have the need for different notification backends, go ahead. Even if you need a notification manager (whatever this is) for this. But don't make code more complicated (sometimes called "test-induced design damage") without need.